I'm wondering why the use of spoilers on XC sailplanes is so low compared to flaps.

As I see it, Flaps:

Have the added benefit of enabling you to vary the Camber of the wing's trailing edge (at the center panels) for thermalling

Are simpler to build into a wing

But are these the only reasons for the widespread use of flaps over spoilers?

Take Care,
John
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Each day Begins with a whole new set of possibilities
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My analysis shows a pretty big penalty for not having camber change. So you need the flaps to win. Also installing spoilers in a composite wing means cutting a hole right where you are most likely to fold the wing. Like servo cutouts, it can be dealt with in the design but it is another thing to worry about. Plus the attention to detail required to make a nice fitting spoiler is in my experience a level above cutting a flap out.

If you need to drop the plane in a tight spot, which happens frequently in some areas, flaps are much easier than spoilers.

Spoilers are of almost no use to a dedicated XC glider -spoilers will only reduce lift and add drag, while flaps allow you to increase lift with little drag penalty at lower deflections for thermalling, and of course larger flap deflections will provide a large increase in both lift and drag. At high flap settings, stall speed is reduced which helps with a slower, steeper landing approach which is helpful in an off field landing situation.

I could sure use some flap or spoilers on XC#8. Last 2 contest I have damaged my plane during landing. At Davis a couple months ago I had great race and completed the course. We stopped our truck in the corner between some trees and power lines. The other side was a huge clear plowed field. Circled and Circled several times. 15 to 20 feet off the deck. Changed directions and ended up hitting a lower power line then the power pole itself. Cal Valley I hit a fence during landing. Tired to keep the plane on the downwind side only to misjudge and find myself about to run into a fence.

We don't get points for landing, but it sure not cool damaging on a perfect XC flying day.